Prions cause neurodegenerative diseases, including CJD, BSE, and scrapie in mammals (Prusiner, “Novel proteinaceous infectious particles cause scrapie” Science, 216:136–144 (1982)). In animals with clinical signs of scrapie, the highest levels of prions are found in the brain and spinal cord, but other tissues, particularly those of the reticulo endothelial system, exhibit substantial prion titers {Eklund, et al. “Pathogenesis of scrapie virus infection in the mouse,” J. Infect. Dis., 117:15–22 (1967); Hadlow, et al., “Virologic and neurohistologic finding in diary goats affected with natural scrapie,” Vet. Pathol., 17:187–199 (1980)). Transmission of prions by oral ingestion of infected tissues is well documented in rodents and more recently in cattle and sheep (Prusiner, et al., “Immunological and molecular biological studies of prion proteins in bovine spongiform encephalopathy,” J. Infect. Dis., 167:602–613 (1993); Carp, “Transmission of scrapie by oral route: effect of gingival scarification,” Lancet, 1:170–171 (1982); Phillips et al., The BSE Inquiry (Stationery Office, London) (2000)). Although BSE prions have clearly been transmitted to teenagers and young adults from cattle[Will, et al., “Deaths from variant Creutzfeldt-Jakob disease,” Lancet, 353:979 (1999); Scott, et al., “Compelling transgenetic evidence for transmission of bovine spongiform encephalopathy prions to humans,” Proc. Natl. Acad. Sci. USA, 96:15137–15142 (1999)), it remains unclear whether cervid prions in North America have transmitted to humans[Spraker, et al., “Spongiform encephalopathy in free-ranging mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), and Rocky Mountainer elk (Cervus elaphus nelsoni) in northcentral Colorado,” J. Wildl. Dis., 33:1–6 (1997)).
The only known constituent of the prion is an abnormal PrP isoform (PrPSc) derived from the normal cell-surface glycoprotein (PrPC), the expression of which is necessary for the production of prions (Büeler, et al., “Mice devoid of PrP are resistant to scrapie,” Cell, 73:1339–1347 (1993); Prusiner, et al., “Ablation of the prion protein (PrP) gene in mice prevents scrapie and facilitates production of anti-PrP antibodies,” Proc. Natl. Acad. Sci USA, 90:10608–10612 (1993)). Western blot analysis demonstrated that in both mice and cattle (Bos taurus), PrPC is expressed in skeletal muscle at a level about 5 to 10% of that in brain (FIG. 1), which is in agreement with earlier reports (Horiuchi, et al., “A cellular form of prion protein (PrPC) exists in many non-neuronal tissues of sheep,” J. Gen. Virol., 76:2583–2587 (1995); Bendheim, et al., “Nearly ubiquitous tissue distribution of the scrapie agent precursor protein,” Neurology, 42:149–156 (1992)).